The invention relates to preparation and application of release coatings for crucibles used in the handling of molten materials that are solidified in the crucible and then removed as ingots, and more particularly to release coatings for crucibles used in the directional solidification of polycrystalline silicon.
Crucibles of fused-silica (quartz) are typically used in directional solidification of polycrystalline silicon. Quartz is chosen primarily for high-purity and availability. There are problems in using quartz, however, as a crucible for the production of silicon by this method.
Silicon in its molten state will react with the quartz crucible that is in contact with it. Molten silicon reacts with quartz to form silicon monoxide and oxygen. Oxygen will contaminate the silicon. Silicon monoxide is volatile, and will react with the graphite components inside the furnace. Silicon monoxide reacts with graphite to form silicon carbide and carbon monoxide. The carbon monoxide will then react with the molten silicon, forming additional volatile silicon monoxide and carbon. Carbon will contaminate the silicon.
The reaction between quartz and silicon promotes adhesion of the silicon to the crucible. This adhesion, combined with a difference in coefficients of thermal expansion between the two materials, creates stress in the silicon ingot, causing it to crack on cooling. It is known in the art that a release coating applied to the inside of the crucible in the area of contact with the ingot can prevent the reaction between silicon and quartz that leads to ingot contamination and cracking. To be effective, the release coating must prevent the silicon from reacting with the quartz crucible, and must not adversely contaminate the silicon either by itself or from contaminants within it.
A variety of materials and techniques are described in the literature, which attempt to solve the problem of reaction and adhesion of the crucible in contact with molten material. For example, U.S. Pat. No. 4,256,530 by Schmid et al., suggests coating the outside of a quartz crucible with a refractory material, to prevent reaction with adjacent carbon components. The coating does not contact the molten silicon. The method of preparing and applying the coating are, however, undisclosed.
U.S. Pat. No. 5,431,869 by Kumar, et. al., describes a multi-component release agent of silicon nitride and calcium chloride for silicon processing using a graphite crucible. The silicon nitride coating is applied as a slurry in an organic binder and solvent. The method of preparation and application are largely undisclosed. It is suggested that the binder can be removed after the coating, but the details are undisclosed. The calcium chloride portion is introduced with the bulk silicon, rather than as a coating, to the silicon-nitride coated crucible. The use of silicon nitride alone is described as unfavorable as a crucible coating for directional solidification of silicon.
U.S. Pat. No. 4,741,925 by Chaudhuri, et. al., describes a silicon nitride coating for crucibles applied by chemical vapor deposition at 1250 degrees Centigrade. U.S. Pat. No. 3,746,569 discloses the pyrolysis formation of a silicon nitride coating on the walls of a quartz tube. The process requires application temperatures at least 800 degrees C, and tempering at 1250 degrees Centigrade. U.S. Pat. No. 4,218,428 by Schmid, et. al., describes a technique of forming a glass layer inside a silica crucible by rapid heating to prevent cracking of silicon during melt-processing.
U.S. Pat. No. 3,660,075 by Harbur et al., discloses a coating of niobium carbide or yttrium oxide on a graphite crucible for melting fissile materials. The niobium carbide is applied by chemical vapor deposition, while the yttrium oxide is applied as a colloidal suspension in an aqueous inorganic solution. Details such as the method of preparation and application are largely undisclosed. U.S. Pat. No. 3,613,633 by Anderson, describes a heated rotating crucible used to hold articles to be coated. The crucible facilitates the containment of an xe2x80x9cevaporantxe2x80x9d which coats the articles therein. The crucible itself is not, however, used to contain molten material.
Reference is made in xe2x80x9cLiquid Encapsulated Bridgman (LEB) Method for Directional Solidification of Silicon Using Calcium Chloridexe2x80x9d, by P. S. Ravishankar, Journal of Crystal Growth, 94 (1989) 62-68, to the coating of a silica crucible with silicon nitride. However, no method is detailed for preparing and applying the coating. Furthermore, the resulting ingot quality using this coating is described as poor, due to particle nucleation leading to poor grain-growth and low solar cell efficiency.
Saito, et. al., in xe2x80x9cA Reusable Mold in Directional Solidification for Silicon Solar Cellsxe2x80x9d, Solar Energy Materials, vol 9, (1983) pg 337-345, and in xe2x80x9cA New Directional Solidification Technique for Polycrystalline Solar Grade Siliconxe2x80x9d, Conf. Record of 15 th PV Specialists Conference, 1981, p 576-580, describes a coating of silicon nitride powder which is brushed onto a quartz, silicon carbide coated carbon or silicon nitride sintered mold. The powder is suspended in an organic solvent, which is evaporated by heating. Methods of preparation and application are not detailed, except that the coating needs to be at least 150 microns thick.
Saito reports, xe2x80x9cThe [silicon nitride] powder was mixed together with a suitable amount of organic solvent, such as liquid polyvinylalcohol, to form a slurry. The slurry was coated by a brush on the inner crucible walls. Then, the crucible was heated in an air ambient at 600 C. for 30 minutes to burn out the organic solvent. The coated layer thus obtained had good mechanical strength against scratching.xe2x80x9d However, no method is detailed for preparing and applying the coating. Brushing, we have found, is a difficult way to obtain a uniform coating.
Scaling a laboratory process such as Saito""s up to production requirements is also problematic. Saito""s crucible was only a few inches across, and contained only 225 g of molten material, while the present technology requires crucibles over two feet across, and contains over 240 kg of molten material. The difference in size and weight makes the physical demands on the coating and the coating process much more profound.
Other publications that mention crucible coatings, usually of silicon nitride, for directional solidification of silicon, but do not discuss methods of preparation or details of application, include: xe2x80x9cHEM Technology for Photovoltaic Applicationsxe2x80x9d, Khattak et al, 6th IPSEC Conference, New Delhi, India, 1992, p 117-124; xe2x80x9cGrowth and Characterization of 200 kg Multicrystalline Silicon Ingots by HEMxe2x80x9d, 26th IEEE PVSC Conference, Anaheim, Calif., Sep. 29-30 1997; xe2x80x9cGrowth of 240 kg Multicrystalline HEM Silicon Ingotsxe2x80x9d, 2nd WCPEC Conference, Vienna, Austria, Jul. 6-10 1998; xe2x80x9cHigh Efficiency Solar Cells Using HEM Siliconxe2x80x9d, First WCPEC Conference, Dec. 5-9, Hawaii, 1994 p 1351-1355; xe2x80x9cCharacteristics of HEM Silicon in a Reusable Cruciblexe2x80x9d, 23rd IEEE PV Specialists Conference, Louisville, Ky., May 10-14, 1993, p 73-77; xe2x80x9cAnalysis and Control of the Performance-Limiting Defects in HEM-Grown Silicon for Solar Cellsxe2x80x9d, Material Research Society Symposium Proceedings, 1995, v 378 p 767-776; xe2x80x9cLifetime Improvement of Multicrystalline Siliconxe2x80x9d, Habler et al., 14 th EPVSE Conference, Barcelona, Spain, Jun. 30-Jul. 4, 1997, p 720-723; xe2x80x9c3D Distribution Study of Impurities into a Polix Ingotxe2x80x9d, Borne et al., 13th European PV Conference, Nice, France, 1995, p 1340-1343; xe2x80x9cStudy and Conditioning of Defect Areas in Eurosil Multicrystalline Siliconxe2x80x9d, Acciarri et al., 13th European PV Conference, Nice, France, 1995, p 1336-1339; and xe2x80x9cSelection of a Crucible Material in Contact with Molten Siliconxe2x80x9d, Revel et al., 5th EC PV SEC, Athens, Greece, Oct. 17-21 1983, p 1037-1042.
Prior art references include specific references to powdered mold release agents for application to crucibles in the directional solidification of silicon. In addition, the use of chemical vapor deposition, solvent evaporation, high-temperature flame treatment, and other expensive and complex means are mentioned for application of crucible coatings. References are made to specific binders and solvents. Although there is a tremendous emphasis in the literature on controlling the purity of the molten material, such emphasis is lacking in the prior art references as to the powder coating process. Silicon Nitride, for example, is available in a variety of phases, purity, and particle size, which may or may not make them suitable for coating.
References are made to mixing, spraying, or brushing for slurries of powdered coatings. There is no mention, however, of a method to mix, spray, or brush the coating in such a way as to control physical properties such as viscosity, foam content, dispersion quality, in order to provide a uniform coating on the crucible and to avoid contaminating the coating in the process of carrying out these steps.
We have discovered, for example, that the use of a specific silicon nitride powder, Baysinid(R), disclosed by Habler et al., in xe2x80x9cLifetime Improvement of Multicrystalline Siliconxe2x80x9d, 14th EPVSE Conference, Barcelona, Spain, Jun. 30-Jul. 4, 1997, p 720-723, in preparing crucible coatings, is heavily aggregated and difficult to disperse, created a poor suspension which was unstable, and caused spraying equipment to clog. This resulted in an extended time required for coating application, and a non-uniform coating which is not reliable in preventing adhesion of molten silicon to the crucible. Ingots were sometimes cracked during operations in which this material was used, due to difficulties related to poor dispersion and clogging. Milling operations using conventional means to properly grind the aggregates to form a stable suspension would contaminate the coating with metal or metal oxide that would contaminate molten silicon.
These examples illustrate how the reviewer of the prior art is led to believe that the selection of specific components and details of their preparation are obvious. We have found, however, that the selection of powders, binders, solvents, and their preparation for applying as a reliable and high-quality coating to a crucible is not in fact obvious, but requires substantial inventiveness to accomplish.
It is an object of this invention to provide a simple, inexpensive coating system and application process for coating crucibles with a release coating for use in a production environment for the manufacture of ingots of polysilicon or other materials. The system will preferably include a coating material having suitable crucible adhesion and ingot release characteristics when applied to a crucible as a release layer for the molten material, and for which there is a powered form of the material available with a suitable particle size and dispersibility for spray application using commercial equipment and conventional methods.
There will be a safe and inexpensive liquid solvent for the coating material, preferably water, in which to suspend the powder, and an organic binder possessing physical and chemical characteristics that facilitate the application of the coating system to crucibles using commercially available spraying equipment. The coating may include additives to improve its quality, make it more sprayable, easier to apply uniformly, and improve its mixing characteristics and control its physical properties. The powder, binder, and solvent are selected and processed such that the resulting final release coating on the crucible does not adversely contaminate the molten material.
It is a further object of the invention to provide a means to remove the solvent from the coating and harden the binder to prevent movement of the coating during subsequent processing, such as by preheating or holding the crucible at a slightly elevated temperature so as to facilitate the evaporation of the solvent and drying of the coating on the crucible after the spraying operation.
It is another object to provide a means to remove the binder by thermal decomposition from the coating and to densify the final coating so as to minimize damage to the coating during subsequent processing of the molten materials, such as by bisque-firing firing the crucible by slowly heating the crucible in oxidizing air to well above a temperature at which the binder material will be readily oxidized dispersed into the air, maintaining the crucible at or near that temperature for a period of time to assure hardening of the remaining layer, and allowing it to cool slowly to room temperature.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by we on carrying out our invention.